1. Field of the Invention
The present invention relates to a process and apparatus for producing ferrochromium having a carbon content ranging from about 0.2 to about 10% from iron-containing chromium ores.
2. Description of the Related Art
Ferrochromium is an alloy composed of from about 20 to about 70% chromium, from about 0.02 to about 10% carbon, from about 0.05 to about 5% silicon, and a remainder comprised of iron and the usual, well-known impurities. Ferrochromium is formed by reduction-by-melting of iron-containing chromium ore, particularly chromium-iron rock, by melting the ore with coal according to the following equation: EQU FeCr.sub.2 O.sub.4 +4 C=Fe+2 Cr+4 CO.
The reduction is effected by melting either a mixture of ore and coke chunks, or a mixture of ore pellets and coke, or a mixture of pre-reduced ore-fine-coke pellets and coke, particularly in a low shaft furnace or in an electric furnace. This results in alloys containing different amounts of carbon.
Ferrochromium is employed as a prealloy in the production of chromium steels. Very frequently high carbon-content ferrochromium alloys are undesirably obtained, but the carbon-content can be reduced by refining the alloys or by refining the chromium steel produced from the alloys. Chromium ores are generally composed of from about 20 to about 50% Cr.sub.2 O.sub.3, from about 20 to about 40% FeO and from about 10 to about 70% rocky matter. It is difficult to separate out the rocky matter before smelting the ores, however, so that the high percentage of rocky matter in prior art reduction-by-melting processes must be separated from the resulting ferrochromium alloys as liquid slag. Since considerable amounts of Cr.sub.2 O.sub.3 are included in the material to be reduced in addition to the high melting point rocky matter, the resulting slags have a high melting point. Thus, in spite of the addition of fluxing agents, melting temperatures of more than 1750.degree. C. must be employed in order to maximize reduction and retrieval of chromium oxide out of the liquid slag in order to keep chromium losses as low as possible and maintain a low slag viscosity. The high temperatures required for such reduction-by-melting processes result in an undesirably high consumption of energy.
In order to be able to perform the reduction and melting processes at low temperatures with the use of carbon as the reduction agent and as the supplier of melting heat, German Pat. No. 3,347,686 proposes adding the slag formers CaO and/or MgO, as well as Al.sub.2 O.sub.3 and/or SiO.sub.2, in such quantities that the rotary furnace slag has a (CaO+MgO) to (Al.sub.2 O.sub.3 +SiO.sub.2) ratio ranging from 1:1.4 to 1:10 and an Al.sub.2 O.sub.3 :Si.sub.2 O ratio ranging from 1:0.5 to 1:5. The reaction product removed from the rotary furnace is comminuted to a particle diameter of less than 25 mm and is then separated by density separation and/or magnetic separation into a coal-containing fraction which is returned to the rotary furnace, at least one metal-containing, slag-rich fraction, and an alloy fraction to be transported into a melting furnace. The alloy fraction is then melted in the melting furnace at a temperature ranging from 1600.degree. to 1700.degree. C. to complete the separation of slag and metal.
To be able to effectively use magnetic separation, it has been further proposed that the reaction product removed from the rotary furnace be cooled to a temperature below the Curie temperature of ferrochromium so that the discharged material takes on ferromagnetic characteristics. Before magnetic separation, the cooled material must be comminuted in a breaker. The metal-containing slag phase and the metal phase are finally charged into a melting furnace into which, limestone dust is added, e.g., 8 kg CaO per minute, in order to reduce the sulfur content and ensure the required desulfuring to a residual sulfur level of 0.01% or less. The above-described process, however, is expensive and requires a corresponding amount of apparatus.